1. Field of the Invention
This invention relates to a control circuit for an inductor motor and, in particular, to a control circuit that functions as a phase shifting network and enables a motor to run more smoothly by producing current and voltage waveforms within the respective motor phases that are smoother, more equal and less subject to harmonic distortion.
2. Disclosure of Related Art
A conventional control circuit 10 for use as a phase shifting network in a two-phase inductor motor is shown in FIG. 2A. Circuit 10 includes a resistor 12 and a capacitor 14 connected in series between a pair of motor phases 16, 18. Each of motor phases 16, 18 includes a motor phase coil 20, 22, respectively. A directional switching device 24 is used to control the sequence of energization of phase coils 20, 22. FIGS. 3A-6A illustrate the various current and voltage waveforms present in motor phases 16, 18 during operation of a motor incorporating circuit 10. In particular, FIGS. 3A and 5A illustrate current and voltage waveforms, respectively, present in motor phases 16, 18 during normal operation of the motor. FIGS. 4A and 6A illustrate current and voltage waveforms, respectively, present in motor phases 16, 18 as a breakdown in motor torque is about to occur. The current and voltage waveforms for phase 16 of circuit 10 are shown in a solid line while the current and voltage waveforms for phase 18 of circuit 10 are shown in broken line. It should be noted that FIGS. 3A-6A illustrate energization of phases 16, 18 in the sequence 16xe2x88x92 greater than 18 (i.e., with the current and voltage waveforms of phase 18 phase-shifted relative to phase 16). As illustrated in FIGS. 3A-6A, the current and voltage waveforms within each individual phase 16, 18 of circuit 10 are subject to relatively large variations in magnitude. Moreover, the magnitude of the current and voltage within phase 16 varies significantly from the magnitude of the current and voltage, respectively, within phase 18. Finally, the voltage in phases 16, 18 is at times subject to a relatively large amount of harmonic distortion as shown in FIG. 6A. These deficiencies result in torque pulses within a motor incorporating circuit 10, thereby causing the velocity of the motor to modulate and the motor to run rough.
There is thus a need for a control circuit for a motor that will minimize or eliminate one or more of the above-mentioned deficiencies.
The present invention provides a control circuit for use as a phase shifting network in a motor such as an inductor motor.
An object of the present invention is to provide a control circuit for a motor that will reduce velocity modulation in the motor and thereby enable smoother operation of the motor.
Related objects of the present invention are to provide a control circuit for a motor that will produce current and voltage waveforms within the motor phases that are smoother, more equal, and less subject to harmonic distortion as compared to the current and voltage waveforms generated by conventional control circuits.
A control circuit for a motor in accordance with the present invention includes a first motor phase having a first resistor connected in series with a first phase coil of the motor. The circuit further includes a second motor phase having a second resistor connected in series with a second phase coil of the motor. Finally, the circuit includes a capacitor connected between the first and second motor phases.
A control circuit in accordance with the present invention smooths the current and voltage waveforms within the motor phase coils by reducing the non-linear characteristics of the motor. First, the control circuit reduces the maximum operating voltage of the motor thereby preventing the magnetic structure of the motor from saturating. Second, the addition of linear impedance devices such as resistors in series with each phase coil makes the motor more linear than the motor alone. A control circuit in accordance with the present invention also equalizes the magnitude of the current in the motor phases coilsxe2x80x94thereby producing a smoother running motorxe2x80x94by equalizing the circuit impedance between the phases of the motor. In a conventional phase shifting network such as circuit 10 in FIG. 2A, the impedance between phases 16, 18 differs by the impedance of a resistor 12 and a capacitor 14. In the inventive control circuit, the difference in impedance between the phases is limited to the impedance of the capacitor (because both phases include a resistor).
The inventive control circuit has several additional advantages as compared to conventional control circuits. First, the inventive circuit results in a lower operating voltage for the motor, thereby allowing the use of wire of various diameters within the motor phases. The use of larger diameter wire can be advantageous because larger diameter wire is easier to wind and to terminate. Second, the inventive circuit enables the motor to run cooler for a given supply voltage as compared to conventional circuits. Finally, the inventive circuit enables a motor to start loads having a larger inertia as compared to conventional circuits.
These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.